To permit effective application of many pesticides by airborne or ground spreading equipment, the active pesticide is impregnated on a solid carrier material which may be diluted to field strength prior to application. The physical properties of the solid formulation, such as ease of flow and retention on the plant, are of the essence. Clays are frequently preferred as the carrier material because of the desirable physical properties of solid pesticide formulation which are based on clay carriers. Some of the clay materials that have been found to be especially suitable as carriers for organic pesticides are the attapulgite and montmorillonite clays. The calcined clays have been considered to possess excellent physical properties as carrier material for organic toxicants. Although the above clays possess excellent flow properties after impregnation with organic toxicants and are readily amenable to impregnation with organic toxicants, the calcined clays unfortunately tend to cause certain types of toxicants, such as thiophosphate ester toxicants, to decompose chemically and to lose their potency when the solid toxicant compositions are stored for long periods in temperate climates or for shorter periods in the tropics. Improvements in stabilizing the clay formulations are continuously being sought.